1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image on a recording medium, an image forming apparatus including the fixing device, and a method of controlling the fixing device.
2. Description of the Related Art
Electrophotographic image forming apparatuses such as copiers and printers have a fixing device that heats and fuses a toner image transferred onto a recording medium to fix the toner image on the recording medium. A belt fixing device and a roller fixing device are widely known as examples of those used in the image forming apparatuses. In the belt fixing device, a toner image on a recording medium is heated and fused by heat from a fixing belt. In the roller fixing device, a toner image on a recording medium is heated and fused by heat from a fixing roller.
For example, the belt fixing device includes a fixing belt, a pressing roller, and a pressure adjusting mechanism. The fixing belt is stretched across and supported by a plurality of rollers such as a fixing roller and a heating roller. The pressing roller is pressed against the fixing roller with the fixing belt interposed therebetween to form a nip portion. The pressure adjusting mechanism adjusts the position of the pressing roller to set the width of the nip portion. When a recording medium is passing through the nip portion between the fixing belt and the pressing roller, a toner image on the recording medium is heated and fused by heat from the fixing belt using the heating roller as the heat source, and pressure is applied to fix the toner image onto the recording medium.
In such a fixing device, the variation in the width of the nip portion (hereinafter, referred to as nip width) results in unstable fixability. The nip width varies according to, for example, the deformation of the fixing roller due to the expansion of a rubber layer of the fixing roller caused by applied heat. As the nip width varies, the amount of heat applied to a toner image on a recording medium that passes through the nip portion changes. As the result, the fixability becomes unstable. In view of this, for example, Japanese Patent Application Laid-open No. 2008-139724 discloses a conventional technology for optimizing the fixing condition by detecting a variation in the nip width.
With the conventional technology, to prevent the fixability from varying due to a deviation from the intended nip width, a variation is detected in the nip width from a change in the position of a tension roller (heating roller). The feeding speed of a recording medium or the temperature of the fixing belt is changed on the basis of the detection result.
According to the conventional technology, a variation in the nip width is detected from a change in the position of the tension roller (heating roller) based on the principles (1) to (3) as follows:
(1) The deformation of the fixing roller changes the tension of the fixing belt.
(2) A change in the tension of the fixing belt changes the position of the tension roller which can be displaced.
(3) The displacement of the tension roller is measured with a range sensor.
It is therefore not possible to detect a variation in the nip width accurately if the elastic characteristic of the fixing belt, i.e., the basis of the foregoing principle (1), changes over time. Accurate detection of a nip width variation is also not possible if the displacement characteristic of the tension roller with respect to a change in the tension of the fixing belt, i.e., the basis of the principle (2), varies. That is, the conventional technology involves mechanical characteristics which are likely to change over time to detect a variation in the nip width. Consequently, it is difficult to continue the detection of a variation in the nip width accurately.